Spoiler alert! The Milky Way and the Andromeda galaxy will collide in four billion years. But alas, the Sun will be a red giant and neither Earth nor the solar system will be here to witness the collision. In 3.75 billion years, the Andromeda galaxy will be so close to the Milky Way that it will fill the night sky. //NASA/ESA/Z. Levay/R. van der Marel/T. Hallas/A. Melliger
It’s rare to find neighbors that don’t meet up every once in a while. So is the case with two of the largest galaxies in the Local Group — the Milky Way and Andromeda. But unlike antisocial neighbors, these two galaxies have a plan to get together. The galactic merger, which is predicted to occur in four billion years, will result in an elliptical galaxy, thus giving a complete makeover to each galaxy’s current spiral structure.
Also known as M31, the Andromeda Galaxy is now 2.5 million light-years away, and the mutual pull of gravity is bringing the galaxy toward us at a speed of 70 miles (110 kilometers) per second. At this rate, we have about four billion years before the collision begins. However, once the Milky Way and Andromeda reach each other, it will take another two billion years for them to completely merge and form into one massive elliptical galaxy.
Furthermore, according to NASA, the third largest galaxy in the Local Group — the Triangulum galaxy (M33) — is also expected to end up in orbit around the merged pair and eventually merge with them at a later date. Schematic of the expected merger between the Milky Way, Andromeda, and Triangulum galaxies. //NASA/ESA/STScI/A.Felid/R. van der Marel
Even though the Andromeda galaxy contains one trillion stars and our own galaxy hosts about 300 billion stars, given the insanely large amount of space in each of the galaxies, the chance of the stars colliding is extremely small.
“When we say the two galaxies are colliding, it's really the dark matter that's mashing up together. Dark matter particles are still an unknown territory for both astrophysicists and particle physicists, but one thing sure is that these particles don't get created in the current universe,” said Tony Sohn, Support Scientist at Space Telescope Science Institute, Baltimore. For now, we will leave the origins of dark matter for the physicists to ponder upon.
The galactic merger will also bring together the supermassive black holes present in the center of each galaxy. These black holes will spiral toward each other and eventually merge when they come so close that they cannot escape each other’s gravitational pull. This merger will be a very violent event that sends massive gravitational waves through the fabric of space-time. Lots of stars will be flung out of the galaxy, and our solar system itself will settle into a new orbit much farther from the galactic center than it is at present.
As scary as this event might seem, don’t fret; galaxies collide all the time! Antennae galaxies (NGC-4038 and 4039) in the Corvus constellation are the nearest and youngest pair of colliding galaxies. Their initial interaction, which began nearly 200 million years ago, is still ongoing today and has resulted in the formation of many globular clusters within the densest regions. This series of illustrations shows how the collision between the Milky Way and the Andromeda galaxy is expected to unfold. The images show how the night sky would appear in: present day (row 1, left); 2 billions years (row 1, right); 3.75 billion years (row 2, left); 3.85 billion years (row 2, right); 3.9 billion years (row 3, left); 4 billion years (row 3, right); 5.1 billion years (row 4, left); and 7 billion years (row 4, right). //NASA/ESA/Z. Levay/R. van der Marel/T. Hallas/A. Melliger
Galactic collisions are commonplace in the universe because, on a local scale, the pull of gravity overpowers the overall expansion of the universe. This means that despite the fact that space-time itself is constantly expanding and pulling objects away from one another — like dots on a balloon that move apart when its inflated — gravity wins out for massive objects that are in close proximity to each other.
“In the grand scheme of the Universe, the distance between our galaxy and the Andromeda is considered very close. In fact, so close that the effect of gravity is much stronger than the expansion of the Universe,” said Sohn, whose research focuses on the dynamics of galaxies.
Fortunately, because gravity is such a weak force, the time scales involved in galactic collisions are absurdly long by human standards. “By the time Andromeda reaches our galaxy,” said Sohn, “the Sun would have evolved into what is called a red giant star, and the surface of this star would have already engulfed the Earth.” In other words, our distant humanoid descendants won’t have seats for the show.
So, until we figure out a way to travel through galaxies (or at least until we can move farther from the Sun), we won’t be able to snag front row ticket for a galactic collision. For now, computer simulations will have to suffice!